Five of histidine's six carbon atoms are derived from PRPP (5-Pribosyl-PP), an intermediate also involved in the biosynthesis of tryptophan and nucleotides. The other carbon originates from ATP. The ATP atoms that are not incorporated into histidine are eliminated as 5-aminoimidazole-4-carboxamide ribonucleotide, wich is also an intermediate in purine biosynthesis.